12.11.2014 15:59, Andrey Semashev wrote: > On Wed, Nov 12, 2014 at 12:26 PM, Alexander E. Patrakov > <patrakov at gmail.com> wrote: >> >> We generally don't need a zoo of resamplers. But you have definitely changed >> something important from an earlier submission by Peter Meerwald so that the >> CPU figure became much better. I guess, that's SOXR_LINEAR_PHASE - that's >> the only obvious change. > > SOXR_LINEAR_PHASE is 0, I added it mostly for documentation purpose, > in case if someone wants to add more modes with different phase > response presets. There is also another difference in that I > explicitly specify interleaved sample formats, although it is > numerically equivalent to those without the _I suffix. > > I think, the essential difference is quality. Peter's patches use > SOXR_QQ, while mine use other presets. I didn't add QQ because I don't > see much point in this mode. Indeed, QQ is just cubic interpolation, worse than speex-float-0. However, I have tried to change that to other values in the old patches, and the result was a good resampler that ate a lot more CPU than speex-float-5. Note, however, that the test was done with a Haswell (year 2013) earlier and with an Arrandale (year 2010) now. > > I also noticed unusual relation between quality and CPU consumption. > You can see that in my results LQ actually appears the most consuming > in some cases. I don't have a good explanation for this, my guess is > that this could be related to different math precision and > optimization. I think the library uses different precision for math on > different quality levels (i.e. double in VHQ, float in HQ, etc.), and > it does have some optimizations for SSE. It's possible that LQ path is > not as optimized. One would probably have to inspect libsoxr code to > tell for sure. > >> Here are the facts, applicable to 44100 -> 48000 Hz resampling, as required >> by some sound cards when playing back CD rips: >> >> * Speex-float-5 never introduces audible distortions, even on >> specifically-crafted testcases at insane volume in an otherwise absolutely >> quiet room. So "even better quality" never makes sense - unless we are >> talking about non-human listeners. >> * Speex-float-1 (the current default) does not introduce audible >> distortions on music that you can buy in shops (or download), but does >> create audible artifacts on specially prepared test cases. >> * SOXR, even at its LQ setting (which, according to prior tests, is good >> enough) and with THIS set of patches, is indeed slightly faster than >> speex-float-1. >> >> Statements about distortion audibility are based on the model described in >> this scientific paper: >> >> http://www.mp3-tech.org/programmer/docs/6_Heusdens.pdf >> >> Distortions caused by removal of high frequencies, as well as any other >> distortions, would have been counted as audible if a person could detect >> that in an ABX test. The nuance here (with "auditory masking" as its >> scientific name) is that the ear becomes less sensitive to very high >> frequencies when there is something else in the air - and in typical music, >> this "something else" definitely exists in a sufficient quantity. > > Thanks for the reference and all the information. > > I didn't do a blind test to detect the audible difference, all I can > say is my own impressions. I have a locally patched PulseAudio 4.0 and > I'm using soxr-vhq preset resampling everything to 96kHz (I have > records in 96kHz that I don't want to resample). The difference with > speex-float-5, which I used previously, is indeed rather subtle. I'd > say, the sound is a bit more clear, instrument separation is better, > especially in bass. But that's all subjective, of course, so it should > be taken with a big grain of salt. In any case, soxr-vhq is clearly > faster than speex-float-5, so at the very least it's a performance > improvement for me. > >> I will recheck the quality separately later today, in order to verify that >> it is still as good as in the previous tests. Please don't merge the patches >> until this is done. > > Thanks, I'd be interested in the results. Will do that after reviewing two work-related (non-audio) patchsets. -- Alexander E. Patrakov
